In automotive applications, a transceiver producing a pulse width coded output signal may be connected to a bus. These transceivers may produce a pulse width coded output to indicate or control local operating conditions for example conditions within an automobile engine. In case of failure of the transceiver, for example by a shorted input, excessive heating may develop, jeopardizing the transceiver and preventing proper operation of the bus. The failed transceiver may load the bus with an erroneous signal, preventing other transceivers from using the bus.
Accordingly, when a transceiver fails such as by a short, it is important to produce an indication of that short and remove the transceiver output signal from the bus. Where the output signal is a current directed into a low impedance connected to a positive voltage or to ground, a failure providing a constant current signal into the bus will cause the bus voltage level to be constant and prevent its proper operation by preventing other transceivers connected to the bus from transferring information.
When the failure is abated and the failed condition is changed to a normal condition, the failed device can automatically be reset and returned to operation. If the defect causing the failure is at or close to its failure condition, a reset signal, placing the transceiver into operation may cause another failure. Accordingly, the failed transceiver should not be placed back in operation until it is in a safe operating range separated from the failure threshold. Where the control signal cycles on and the transceiver failure indicator is near its threshold failure condition, the occurrence of an on reset signal turning the transceiver on again when at that threshold failure condition, may cause a premature failure. Accordingly, it is important to prevent the transceiver or element from being turned on even in the presence of a reset signal until its operating condition is within its safe operating range, within the threshold failure level with a margin of safety, indicating safe operation. To prevent the failed transceiver from operating before a reset signal is given, a reset control on signal should be cycled through one complete cycle from off to on, before the failed unit is allowed to transmit signals to the bus again.
When a signal indicating excessive temperature is received the output signal of the transceiver may be removed from the bus allowing other transceivers which are connected to the bus to communicate normally. However, when the signal indicating excessive temperature on the failed transceiver is removed by cooling, the device can be reset, generating an erroneous signal on the bus, if the failed transceiver has not cooled to a safe operating range.